Yellow Stains on Bathroom Countertops or any other countertops can usually be easily removed with some hydrogen peroxide.
You can pour some hydrogen peroxide into a bowl and then dip a washcloth into the bowl of hydrogen peroxide and then take the hydrogen peroxide soaked cloth and wipe it over the yellow stain on the countertop and it should start coming off.
If that doesn't work then you could mix some baking soda with some water to make a paste and apply the paste to the stain on the countertop.
Then scrub the stain with a cloth or a toothbrush or another scrub brush and it should come off or you can use some vinegar with the baking soda to help it work out the stain better.
Hydrogen peroxide usually gets the yellow stain off my countertops pretty easily though.